244 



cumstances," read at the last meeting of the Royal Society. 

 By Professor Forbes. 



The object of this paper was to develope an application of the 

 fact communicated by the author on the 21st January. It was 

 then remarked, tliat the discovery that steam in a certain stage of 

 condensation is deeply red-coloured for transmitted light, seemed 

 to offer a probable solution of a difficulty which has never yet been 

 fairly met, namely, the red colour of clouds at sunset, and the red- 

 ness of light transmitted thrfiugh certain kinds of fogs. 



A pretty full history of theories proposed to account for the co- 

 lours of the atmosphere was first given ; it was obtained in almost 

 every case from an examination of the original authorities. These 

 theories were reduced under three general heads, exclusively of 

 that of Gothe, and of most writers before Newton, that the blue 

 colour of the sky results from a mixture of light and shade ; and 

 that of Muncke, that that colour is merely subjective, or arises from 

 an ocular deception. The remaining theories are : 



(1.) That the colour of tiie sky is that transmitted by pure air, 

 and that all the tints it displays are modifications of the reflected 

 and transmitted colours. This is more or less completely the opi- 

 nion of Mariotte, Bouguer, Euler, Leslie, and Brandes. 



(2.) That the colours of the sky are explicable by floating va- 

 pours acting as thin plates do, in reflecting and transmitting com- 

 plementary colours. This is the theory of Newton and most of 

 Lis immediate followers, and more lately of Nobili. 



(3.) On the principle of opalescence and of specific absorption, 

 depending on the nature and unknown constitution of floating par- 

 ticles. This head is intended to embrace the various opinions of 

 Melvill, Delaval, Count Maistre, and Sir D. Brewster. 



To the last named philosopiier, however, the merit is due of 

 having conspicuously turned attention to the important, com- 

 plex, and hitherto unexplained phenomena of absorption, which 

 he has proved to be totally inconsistent with Newton's theory of 

 the colours of Nature, (considered as those of thin plates) ; and he 

 has farther demonstrated the inapplicability of it in the case of the 

 colours of the atmosphere, by shewing that their constitution is 

 wholly distinct from that which any moditication of Newton's theory 

 would assign, by a series of experiments of which as yet the re- 

 sults only are announced. 



Since, then, the constitution of the atmospheric colours analyzed 



